1. Field of the Invention
The invention relates to a method and system for de-interlacing, and more particularly, to a method and system for four fields de-interlacing with motion detection, motion compensation, directional edge interpolation, 3:2 pull-downed detection, and 3:2 pull-downed recovery.
2. Description of the Prior Art
Modern computer systems are not constrained by data transfer rate required for the non-interlaced format to the same extent as were early television systems. However, the use of interlaced display formats for many consumer video products is common and unlikely to disappear soon. In order to display this interlaced material on a non-interlaced computer display there is a need for de-interlacing.
De-interlacing has become an important technique for scanning rate conversion in nowadays. Interlaced sequences are usually transmitted to the audiences by 60 fields per second. In order to display these interlaced sequences on the progressive devices (e.g. Computers, HDTV), the progressive devices ought to reconstruct the missing field. If de-interlacing is not done perfectly, audiences will suffer uncomfortable visual artifacts. These kinds of defects such as edge flicker, line crawling will cause critical distortion m the output frames.
Progressive devices commonly use a non-interlaced video display format, also called a progressive scan. That is, an entire frame is scanned line-by-line, typically from top to bottom. The process repeats and re-scans the entire frame at a given refresh rate, for example, 75 Hz. In contrast, many sources of consumer video, such as television signals, use an interlaced display format. Interlaced systems interleave two or more fields to display an entire frame. A typical U.S. NTSC television signal uses two fields, each with video data for every other horizontal line of the entire frame, and consecutively scans the two fields 30 times per second. The audience perceives the resulting image as a blend of the two individual fields with an effective refresh rate of 60 Hz. If instead, the entire frame was scanned progressively at 30 Hz, the audience might perceive a flicker of the displayed image. The flicker is much less apparent with higher refresh rates such as 60 Hz. The interlaced format thus reduces flicker without requiring a doubling of the data transfer rate needed to update the entire frame at 60 Hz. It calls the 3:2 pull-down. While 3:2 pull-down method transforms sequences from 24 frames per second into 60 fields per second, it generates fields patterns like “EOE OE” or “OEO EO” (E means even field, O means odd field) where fields together like “EOE” or “OE” means they came from the same frame. Other interlaced video formats are also common, such as the 50 Hz phase alternation line (PAL) system used in Europe.
The timing of the two or three fields generated from a frame is the same and the fields generated by way of this are called the fields in progressive video. On the contrary, if the timing of each field is different, then the fields are called the fields in interlaced video. The images of a motion are on the same location if the fields are at the same timing in the fields in progress video, but they are on the different locations in the fields in interlaced video.
A frame is combined by an odd line field and an even line field. In other words, interleaving the odd and even lines combines the frame. Thus, the method for de-interlacing a field to reconstruct a frame is to interleave the odd and even lines of the field and a complement field. The de-interlacing methods are classified as three categories: intra-field interpolation, inter-field interpolation, motion adaptive interpolation.
As mentioned above, inter-field interpolation is a simple de-interlacing method that it just combines two interlaced fields into one progressive frame. Inter-field interpolation may suffer severe visual quality degradation if there are lots of fast motion images in those field sequences. So inter-field interpolation is not considered to be a good method for de-interlacing a 60fps fast-motion field source. Another popular linear technique is intra-field interpolation, which constructs a new field by the intra-field interpolation to combine with the current field. The audiences may not feel uncomfortable while watching sequences de-interlaced by the intra-field interpolation. But there is a disadvantage while adopting intra-field interpolation, the vertical resolution is halved due to the interpolated new field: the output frame will be blurred after the combination of the current field and the interpolated new field. Motion Estimation can obtain good image quality, but it also costs a lot of time. Thus a motion adaptive interpolation with all kinds of advantages and precise motion detection is preferred.